Well, I am certainly feeling warm. But that is because of the burning sense of shame that is travelling from the tips of my toes all the way to the top of my hooded head, and creating a furnace of embarrassment around my body. If I had stripped naked, smothered myself in custard and performed the Gangnam Style dance on the pavement, I could not have felt more self-conscious.

However, I am not wearing custard – I am sporting something even more ludicrous: a onesie.

This is the fashion world’s two‑fingered salute to square bears such as me who choose to wear a jacket and tie to work. There is no other way to describe it but as a romper suit for grown‑ups. Designed for those for whom buttons are a struggle and tying laces is a challenge, the onesie is an item of clothing perfectly suited to our infantile age, where adults suck milky coffee out of beaker-like cups and prime ministers play Angry Birds.

And it has exploded in popularity.

I first came across them a few years ago, when they were the novelty rival to the “slanket” – a blanket with arms designed for couch potatoes. Back then, onesies were known as Snuggle Suits and were clearly a joke. They were found at the back of gimmicky magazines, alongside bath pillows and solar‑powered garden gnomes.

Big, bulky and 100 per cent polyester, they cost about £30 and were designed to be worn in the privacy of one’s own home. They were not fashion items; they were alternatives to central heating. Asda and the catalogue company House of Bath were among the first to sell them – hardly places that you associated with the catwalks of Milan or the celebrity-trod pavements of Chelsea.

That has all changed, however. A raft of mainstream fashion retailers, including the two grand old dames of the high street, Marks & Spencer and John Lewis, have started selling onesies. And a number of celebrities have been photographed wearing the extraordinary garments – not on the pages of some glossy gossip rag, welcoming the world into their lovely gold-tapped, leather-lounged home. No, out in the street. In public.

Retail figures suggest that this is more than a passing fad. John Lewis said its sales are growing at a rate of 73 per cent a week. Asda has sold out of its Cheeky Chimp version. Marks & Spencer has ordered 15 times more of the things than last year and New Look sold 75,000 in September alone.

OnePiece, a Norwegian company, has been credited with shepherding the onesie from the pages of the Innovation Catalogue to the fashion spreads of Grazia. It offers an astonishing 104 different styles, from pink-and-white “lollipop stripes” to more subtle navy blue, with white edging. These can be yours for around £140 a pop.

Lisa Dwan, who brought the OnePiece to Britain, says: “It has magical powers. I went from 'I’ll never wear that’ to 'OK, only indoors’ to 'I don’t give a damn, it’s so comfortable I am never taking it off.’ I like to think that I still have some self-respect, but I’ve worn mine in nightclubs, with sexy lingerie underneath. It is just so comfortable – it is like stepping into a teddy bear’s womb.”

Cheryl Cole has championed the pyjamas-as-streetwear look. So, too, Peaches Geldof, Sadie Frost, Robbie Williams and Jennifer Ellison, former Brookside actress and semi-finalist of Dancing on Ice, and the cast of Made in Chelsea. Even quite respectable celebrities, such as Olympic diver Tom Daley, and proper A-list ones, Brad Pitt and Kate Moss, have merrily been seen in public looking like they are off to a baby yoga session.

I’m told that the look has made it as far as the studies of Eton, a school that helped put the stiff into the British upper lip and the starch into stuffed shirts. Baffling.

I decided it was time to discover whether the classless item of clothing really is going to become this year’s winter must-have, as the retailers predict. So, I hit the streets of Westminster to give my OnePiece Lillehammer a test run. The pattern is meant to be sexy-Scandi, a sort of all-in-one Sarah Lund jumpsuit. But I feel as if I am covered in QR codes.

Within 20 yards of stepping out of the office, I am yearning for the comforting assurance of my Savile Row suit and M&S tie. My inner Mr Pooter wants to jump under the first passing number 11 bus, rather than be seen by anyone I might know.

But it soon becomes clear that London has lost its last flicker of Olympic glow and returned to being the most blasé capital in Europe. Most pedestrians either don’t care that a fashion homicide is being committed under their noses, or think I am a lunatic who should be given a wide berth.

Waiting at a bus stop, I finally arouse enough curiosity for Orlando, a fund manager, dressed in a tailored pinstripe suit, to ask about my OnePiece. “It’s quite entertaining,” he says. When I tell him that the kids are now wearing it out on the town or even to the office, he looks a little surprised. “On a skiing holiday, I can see it would be fun, but I don’t think my wife would allow me through the front door if I returned from work looking like that.”

Lucia stops me, too. She owns a £30 leopard-print onesie from Primark. “I’m a big fan, but not out in the street. Also, I don’t like that you can’t go to the loo in them.”

This may be true but as I wander the streets in my mortifying onesie, I realise that Dwan is right – it really is remarkably comfortable. Unlike the cheap versions, which envelop you in a polyester fug, this is made from thick tracksuit material and gives me a reassuring hug as I sashay down the street. I may look like a pillock wrapped in bar code, but I am warm and dangerously relaxed.

The all-in-one has a long history. Winston Churchill asked Turnbull & Asser, the Jermyn Street tailor, to knock up a “siren suit” for him so that he could work in comfort during the night – be it in his Downing Street study or down in the air raid shelter. His red and green numbers look remarkably like a velour Juicy jumpsuit of today.

Churchill, of course, had Nazi bombers as his excuse for a lapse in sartorial standards. I have nothing save the disintegration of Western civilisation. Loungewear in public has been gathering pace in recent years, and its manifestations are increasingly worrying: England rugby players wear flip-flops and Dr Dre headphones when they fly abroad on a world tour, Uggs are worn on a night out, slippers on the school run.

Sporting a onesie in public is the final apotheosis of this trend. It may be comfortable – but so are incontinence pads. It must be challenged.

The British Empire collapsed around the time that shirt studs and detachable collars fell from fashion. If we allow the onesie to become as widely acceptable as the shell suit was in the Eighties, we are surely on the road to fleecy, snuggly ruin.